1. Field of Invention
The present invention relates to an active matrix substrate, an electro-optical device and electronic equipment. More specifically, the invention relates to a construction of an active matrix substrate ideally used with a liquid crystal light valve mounted on a projection display unit.
2. Description of Related Art
A related art liquid crystal light valve is an optical modulating device to be mounted on a projection display unit, such as a liquid crystal projector. The liquid crystal light valve is primarily constructed of a pair of substrates that are disposed to oppose each other with a liquid crystal layer sandwiched therebetween, and electrodes to apply voltages to the liquid crystal layer. Normally, in the related art, the liquid crystal light valve uses an active matrix liquid crystal cell, and it is advantageous to achieve higher definition of images.
As the methods of driving a liquid crystal light valve, reversal drive methods, including dot reversal, line reversal and field reversal, have been used in the related art to reduce or prevent seizure or deterioration of liquid crystal.
Each of the above inversion drive methods has advantages and disadvantages. In the case of the dot reversal or the line reversal, voltages of opposite polarities are applied to the pixel electrodes of adjoining dots, so that a horizontal electric field is generated between the adjoining dots, and light leakage attributable to disclination caused by the horizontal electric field may occur. As mentioned above, since higher definition is required of liquid crystal light valves, the leakage of light leads to deteriorated contrast or aperture ratio, which is a major cause of degraded display quality. From this viewpoint, it is required to adopt a field reversal drive system free of the occurrence of horizontal electric fields.
However, in the related art, it has been impossible to adopt the field reversal drive method in the construction of a related art active matrix substrate, because of the following reason.
In the field reversal drive, when attention is focused on, for example, a single data line, image signals (voltages) of the same polarity are written in a certain one field with respect to all dots to which a signal from the foregoing data line is supplied. Then, the moment the next field is reached, the polarity of an image signal to be supplied to the foregoing data line is reversed. At this time, on the scanning line side, if scanning is carried out from the top to bottom of a display region, the image signal supplied to the foregoing data line is immediately written at upper dots in the display region. On the other hand, lower dots remain in a prolonged state where pixel electrodes retain an image signal written in the preceding filed, while an image signal of the opposite polarity from that is applied to the data line. During this period of time, coupling between the pixel electrodes and the data line takes place, thus posing a problem in that the potentials of the pixel electrodes vary due to the influences of the data line at the lower dots of the display region, resulting in deteriorated display quality.
The present invention addresses the above and/or other problems, and provides an electro-optical device, such as a liquid crystal device, that permits improved display quality to be achieved by adopting field reversal drive, an active matrix substrate used therewith, and electric equipment.
To this end, an active matrix substrate in accordance with the present invention includes: a substrate body equipped with a plurality of data lines and a plurality of scanning lines provided such that they cross each other, a plurality of thin-film transistors electrically connected to these data lines and scanning lines, and a plurality of pixel electrodes electrically connected to the plurality of thin-film transistors, respectively. Gate electrodes constituting the thin-film transistors and the scanning lines are formed in separate layers, and electrically connected via contact holes penetrating an interlayer insulating film between the gate electrodes and the scanning lines. A layer constituting the scanning lines is positioned above a layer constituting the data lines but under a layer constituting the pixel electrodes, and a pattern of the scanning lines, a pattern of the data lines and a pattern of the pixel electrodes are partly overlapped in a top plan view.
More specifically, in the active matrix substrate in accordance with the present invention, the gate electrodes constituting thin-film transistors (hereinafter xe2x80x9cTFTsxe2x80x9d) are not formed integrally with the scanning lines. Instead, the gate electrodes are independently formed using a different layer from that of the scanning lines, and the gate electrodes and the scanning lines are electrically connected via a contact hole. In the sectional structure, the layer constituting the scanning lines is positioned between the layer constituting the data lines and the layer constituting the pixel electrodes, and the pattern of the scanning lines partly overlaps the pattern of the data lines and the pattern of the pixel electrodes in a top plan view. Structurally, therefore, the portion wherein the scanning lines overlap with the data lines and the pixel electrodes functions as a shielding layer to block the coupling between the pixel electrodes and the data lines described with regard to the related art. This reduces or minimizes the chance of variations occurring in the potentials of pixel electrodes caused by the influences of the data lines at any location in the display region. Thus, an electro-optical device employing the active matrix substrate permits the field reversal drive. The use of the field reversal drive makes it possible to obtain an electro-optical device, such as a liquid crystal device, with, for example, a higher contrast and a higher aperture ratio. Moreover, since the scanning lines required for the active matrix substrate are used as the shielding layer, no separate pattern functioning solely as the shielding layer is added, so that the pattern construction will not be particularly complicated.
In the active matrix substrate in accordance with the present invention, the scanning lines are preferably formed of a material including a metal. Further preferably, the data lines are also formed of a material including a metal. The term xe2x80x9ca material including a metalxe2x80x9d means that the layer may be formed of a single metal layer or a laminated film containing a metal film.
For instance, a liquid crystal device mounted as an optical modulating device on a projection display unit is irradiated with far more intense light than that applied to a direct view liquid crystal display equipped with, for example, a backlight. At this time, when light is applied to a TFT provided as a pixel switching element, light leakage current passes between source and drain regions, leading to a problem in which the characteristics of the TFT deteriorate or the TFT malfunctions in an extreme case. For this reason, the related art creates a light shielding film on the active matrix substrate. A case may exist where the light shielding film is formed separately from various wires, or scanning lines are formed of a material, e.g., WSi (tungsten silicide) exhibiting high light shielding performance, to use them as the scanning lines serving also as a light shielding film in order to simplify the substrate configuration. However, especially in the latter case, the material, such as WSi, exhibits good light shielding property, while it disadvantageously has a high sheet resistance, approximately 5xcexa9, leading to a problem of degraded display quality due to signal delay in the scanning lines. In addition, the light shielding film is disposed only in one direction, resulting in inadequate restraint of light leakage current.
In contrast to the above, in the active matrix substrate according to the present invention, if the scanning lines are formed of a material, including a metal, such as aluminum, then the sheet resistance will be sufficiently lower, approximately 0.1xcexa9, than that of the SWi. Hence, even when the device is driven at a high frequency of 60 Hz or more, the degradation in the display quality attributable to signal delays in the scanning lines can be restrained. Similarly, forming the data lines with a material including a metal, makes it possible to restrain deterioration in the display quality caused by signal delays in the data lines. Especially if both scanning lines and the data lines are formed of a material including a metal, then these scanning lines or the data lines function as a light shielding film and the light shielding film is disposed in a grid pattern, allowing light leakage current to be satisfactorily reduced or restrained.
The gate electrodes may use diverse types of materials. However, it is preferably formed of polycrystalline silicon.
If the scanning lines and the gate electrodes are integrally formed, then using a metal to form the scanning lines automatically means that the gate electrodes are also formed of the metal. If, however, the gate electrodes are formed of a metal, then the metal may diffuse into a gate insulating film during, for example, a heating step in the manufacturing process, and a problem, such as unstable device characteristics of the TFT, may arise. In the active matrix substrate according to the present invention, the scanning lines and the gate electrodes are constructed of separate layers, allowing the materials for these two to be individually selected. Hence, even when the scanning lines are formed of a metal to prevent delays in wiring, the gate electrodes may be formed of polycrystalline silicon. As a result, the device characteristics of the TFT can be stabilized, permitting enhanced reliability to be achieved.
Preferably, a light shielding film extending in a grid pattern in the directions along the scanning lines and the data lines is provided, through the intermediary of an interlayer insulating film, below a semiconductor layer constituting channel regions of the TFTs.
As described above, when the scanning lines or the data lines are formed of a metal with high light shielding performance, these scanning lines and data lines function as a light shielding film, making it possible to block light entering from above the substrate into the TFTs. In addition, when a light shielding film extending in a grid pattern in the directions along the scanning lines and the data lines is provided under the semiconductor layer constituting the channel region of the TFT, it is possible to block the entry of light from below the substrate into the TFT.
Furthermore, a storage capacitor electrode to form a storage capacitor between itself and the semiconductor layer constituting the channel region of the TFT is provided, and the storage capacitor electrode is formed of the same layer as the layer constituting the gate electrode.
With this arrangement, an image signal (voltage) written to a pixel electrode will be retained further securely, and the storage capacitor electrode constituting the storage capacitor can be formed at the same time when the gate electrode is formed, thus reducing or preventing the manufacturing process from becoming complicated.
Furthermore, when the storage capacitor electrode and the light shielding film (the light shielding film under the semiconductor layer) are provided, the storage capacitor electrode and the light shielding film are preferably electrically connected through a contact hole penetrating an interlayer insulating film between these storage capacitor electrode and the light shielding film.
With this arrangement, the storage capacitor electrode and the light shielding film share the same potential, and they are positioned above and under the semiconductor layer of the TFT, respectively, through the intermediary of the interlayer insulating film, allowing the double-stage storage capacitor to be formed above and under the semiconductor layer. As a result, the storage capacitor value in a certain occupied area can be increased, and the display quality can be enhanced.
Alternatively, the area occupied by the storage capacitor can be decreased to obtain a predetermined storage capacitor value, and the aperture ratio can be increased. Moreover, since the potential of the light shielding film positioned below the TFT can be fixed, the operational stability of the TFT can be enhanced.
The scanning lines may be configured to have portions that protrude along the data lines from the trunks of the scanning lines. Similarly, the data lines may be configured to have portions that protrude along the scanning lines from the trunks of the data lines.
With this arrangement, a portion that is branched from the trunk of a scanning line or a trunk of a data line and protrudes in a perpendicular direction can constitute a part of the light shielding film, making it possible to further improve the light shielding performance with respect to the TFT.
Alternatively, a relaying conductive film formed of the same layer as the layer constituting the data lines may be provided, and the semiconductor layer and the pixel electrodes may be electrically connected through the relaying conductive film. For example, if the pixel electrodes are formed of a transparent conductive film of, for example, indium tin oxide (hereinafter xe2x80x9cITOxe2x80x9d) or the like, then preferably, at least the upper surface of the relaying conductive film is formed of a material permitting ohmic connection with the transparent conductive film.
With this arrangement in which the relaying conductive film is provided, even when the distance between the semiconductor layer and the pixel electrode layer is long, e.g., about 1 xcexcm to about 2 xcexcm, these two layers can be successfully connected via two or more contact holes having a relatively small diameter, while reducing or avoiding the technological difficulty of connecting the two layers via a single contact hole. Thus, the aperture ratio of pixels can be increased. Although the semiconductor layer constituting the TFT is usually extremely thin, it is useful to reduce or prevent over-etching when opening contact holes. Especially when at least the upper surface of the relaying conductive film is formed of a material that permits ohmic connection with the transparent conductive film, the contact resistance can be decreased.
Furthermore, a drive circuit to drive the scanning lines or the data lines may be provided, and a gate line for the TFT constituting the drive circuit may be formed by using the layer constituting the scanning lines, the layer constituting the data lines, or the layer constituting the gate electrodes.
The TFTs corresponding to the dots in the display region must be formed such that the scanning lines and the gate electrodes are constituted in separate layers and electrically connected through the contact holes in order to obtain the aforesaid advantages according to the present invention. Meanwhile, when the drive circuit is provided on the active matrix substrate, there are no particular restrictions on the TFTs making up the drive circuit. Hence, the gate lines may be formed by selecting one of the layer constituting the scanning lines, the layer constituting the data lines, or the layer constituting the gate electrodes.
Preferably, the upper surface of a region where the scanning lines or the data lines are formed is positioned at a higher level than the upper surface of a central portion of a region where the pixel electrodes are formed.
As described above, disclination may occur and light leakage may happen in the boundary of adjoining dots. To reduce or prevent this problem, the aforesaid arrangement is advantageous in that the region wherein the scanning lines or the data lines are formed, that is, the peripheral portions of the dots, is shaped like an embankment, and the liquid crystal layer of the peripheral portions of the dots is thinner than that of the central portion of the region where the pixel electrodes are formed, that is, the liquid crystal layer of the central portions of the dots. Thus, in the peripheral portions of the dots, the intensity of the vertical electric field applied to the liquid crystal layer is higher, providing an advantage of reduced disclination.
When the above arrangement is adopted, the upper surface of the interlayer insulating film in contact, at the bottom side of the data lines, with the data lines may be planarized. Alternatively, the upper surface of the interlayer insulating film in contact, at the bottom side of the scanning lines, with the scanning lines may be planarized.
As described above, when the peripheral portions of the dots are shaped like embankments, the planarized upper surface of the interlayer insulating film in contact with the data lines at the bottom side of the data lines makes it possible to adjust the height of the embankment by use of the film thickness of the data lines or the scanning lines. This allows the disclination to be securely controlled. Furthermore, the planarized upper surface of the interlayer insulating film in contact with the scanning lines at the bottom side of the scanning lines makes it possible to adjust the height of the embankment only by use of the film thickness of the scanning lines.
Alternatively, a recessed portion is provided in the region of the substrate body wherein the TFTs are formed.
With this arrangement, some of the TFTs may be embedded in the recessed portion of the substrate body, and contrary to the above case, the upper surface of the active matrix substrate can be further planarized.
An electro-optical device in accordance with the present invention includes the aforesaid active matrix substrate in accordance with the present invention.
With this arrangement, the provision of the active matrix substrate in accordance with the present invention permits the use of the field reversal drive, making it possible to obtain an electro-optical device, such as a liquid crystal device, with a high contrast and a high aperture ratio.
An electronic equipment in accordance with the present invention includes the aforesaid electro-optical device in accordance with the present invention.
This arrangement makes it possible to accomplish electronic equipment provided with a display unit featuring high display quality.